This invention relates to sampling methods used in analytical instrumentation.
Gas cells are used in spectroscopy for containing gas samples during analysis. They consist of a containment vessel with optically transparent windows and may include mirrors. In spectroscopy, the absorption of the analyzing beam is proportional to the distance the beam travels through the sample and inversely proportional to the concentration, so gas cells are built with different lengths depending on the types of samples and concentrations that are being analyzed. The distance the beam travels through the sample can be increased through the use of mirrors to reflect the analyzing beam for multiple passes. The length the analyzing beam travels through the sample is referred to as the path length. Gas cells can be used in a continuous flow mode for monitoring or in stop flow mode for static analysis of a sample. Depending on the type of samples being analyzed, they may be heated above ambient temperature in order to reduce condensation. Keeping the optical elements of the cell, the windows and/or mirrors, clean is of utmost importance in order to achieve accurate results. The goal for the gas flow through the cell is to avoid areas where the sampled gas is not readily exchanged. This insures the total path length being analyzed reflects the actual sample being presented to the cell. FIG. 2 shows a typical prior art cell in which two ports for flowing the gas are provided, an inlet port 4 and outlet port 5. The TGC-S10 cell, manufactured by Hayrick Scientific Corporation, Ossining, N.Y., is one example and similar cells are available from other manufacturers. In current practice the inlet and outlet are placed toward the ends of the cell for proper flow. When being used to sample dirty samples such as from a Thermogravimetric Analyzer or pyrolysis, which can contain a number of gases and particulate mater, the position of the inlet near a window or mirror can lead to contamination.
In order to minimize the internal volume of the cell, the cell is sometimes built in the shape of the infrared beam, typically in the shape of two cones placed point to point for use with the focused beam typical for many spectrometers. These cells are heated by placing a band heater in the center of the cell or by a blanket heater covering the entire cell. The typical temperature range is from ambient to 250° C. Cells can be composed of a number of materials including stainless steel, coated aluminum, HASTELLOY®, glass and others. The windows for the cell are composed of a number of materials depending on the spectroscopic wavelength to be used. For Infrared use common materials are KBr, NaCl, KCl, ZeS, among others, while quartz and glass can be used for visible analysis.